Four major factors dramatically contribute to the failure of known threaded fasteners; namely: inappropriate preload; creep in all the components of the joint established by the threaded fastener; environmentally generated off-torque; and elevated and differential temperature.
Various designs have been proposed to solve these deficiencies, however failure of threaded joints continue to persistently occurs in almost every industry leading to loss of life, damage to equipment and other property including loss of product, production delays, and environmental disasters.
Wedge fasteners have been proposed in various forms to counteract a vibration or shock generated off-torque. Typically, such wedge fasteners utilise a washer to engage a surface of the work-piece with increased friction. However, the configuration of the washer can have the effect of damaging the work-piece surface with which it engaged, as well as causing accelerated creep at an increased magnitude and preventing accurate preloading. A design that operates without a frictional surface has been proposed in U.S. Pat. No. 4,793,752.
However, it requires an extensive flange for its operation. A design that employs ratchet teeth for easy preloading is disclosed in U.S. Pat. No. 4,034,788. It, however, requires the use of locking teeth that embed themselves into a work-piece.
A self-centring feature has been proposed for fasteners disclosure in U.S. Pat. No. 3,241,589, U.S. Pat. No. 3,926,237 and U.S. Pat. No. 4,283,091. Loading of the proposed fasteners is likely to be highly uncertain due to work-piece embedding teeth and linear contact between a washer and a threaded fastener.
In certain applications, it is important for fasteners to be loaded to a specified torque in order to ensure that a desired holding force is developed within the joint. Precision loading of a fastener to achieve a prescribed torque can be difficult to achieve. The most common method currently in practice, which has an accuracy of 25%, involves use of a torque wrench. Angle controlled tightening, which has a 15% accuracy, requires skilful labour to be properly applied. An example of such is disclosed in U.S. Pat. No. 5,165,831. The method bears some of the deficiencies of the torque wrench as it requires in its first step the application of a snug torque. ‘Turn of the screw ’ method is disclosed in U.S. Pat. No. 3,851,386, involving finger tightening in the first step and alignment with predetermined position ‘that looks and feels right’ but which is less than elongation of failure. Although providing a highly satisfactory solution, a fastener incorporating a scale, such as the fastener disclosed in U.S. Pat. No. 5,411,008, is limited to the application on which it is employed.
Precision loading is particularly important where fasteners cooperate in a circular array. For securing wheels, a ‘quarter turn’ method is in common use where a quarter turn at a time is applied in a crisscross sequence. For flanges, several passes of a ‘four point star’ are often applied. More accurate loading can be achieved by the method disclosed in U.S. Pat. No. 5,278,775.
In relation to motor-aided applications, flange nuts mounted on a spindle of a power tool suffer thread profile permanent deformation when subject to high torque. The effect is particularly severe in tools of a larger diameter, such as cutting or grinding discs. Hand operated nuts that correct the problem have appeared recently on the market. A variety of designs have been proposed (for example as in U.S. Pat. No. 5,899,648), but component parts numbering usually in excess of 20 and complexity of manufacturing render such hand-operated nuts uneconomical for separate sale and they are usually sold as a part of a power tool.
It has been recently acknowledged that addition of a cone spring washer to the fastener assembly effectively counteracts creep. Various designs have been proposed, several examples of which can be seen in U.S. Pat. Nos. 5,547,326 and 5,597,279. None of the proposals, however, considers how such a washer aids off-torque after a certain amount of creep lifts the central portion off the work-piece.
It is against this background, and the problems and deficiencies associated therewith, that the present invention has been developed.